Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic projection apparatus is disclosed in which a liquid supply system provides a liquid between the projection system and the substrate. An active drying station is provided to actively remove the liquid from the substrate W or other objects after immersion of all or part of a surface of the substrate W or other objects.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patent application Ser. No. 10/847,661, entitled “Lithographic Apparatus And Device Manufacturing Method”, filed on May 18, 2004. The content of that application is incorporated herein in its entirety by reference.

FIELD

The present invention relates to a lithographic apparatus and a device manufacturing method.

BACKGROUND

A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.

It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective NA of the system and also increasing the depth of focus.)

However, submersing the substrate or substrate and substrate table in a bath of liquid (see for example U.S. Pat. No. 4,509,852, hereby incorporated in its entirety by reference) means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors and turbulence in the liquid may lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection system and the substrate using a liquid supply system (the substrate generally has a larger surface area than the final element of the projection system). One way which has been proposed to arrange for this is disclosed in PCT patent application publication WO 99/49504, hereby incorporated in its entirety by reference. As illustrated in FIGS. 6 and 7, liquid is supplied by at least one inlet IN onto the substrate, preferably along the direction of movement of the substrate relative to the final element, and is removed by at least one outlet OUT after having passed under the projection system. That is, as the substrate is scanned beneath the element in a −X direction, liquid is supplied at the +X side of the element and taken up at the −X side. FIG. 6 shows the arrangement schematically in which liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. In the illustration of FIG. 6 the liquid is supplied along the direction of movement of the substrate relative to the final element, though this does not need to be the case. Various orientations and numbers of in- and out-lets positioned around the final element are possible, one example is illustrated in FIG. 7 in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the final element.

Clearly the presence of liquid in the immersion lithographic projection apparatus raises difficulties not present in conventional lithographic apparatus. For example, sensors such as interferometers for measuring the position of a substrate table which supports the substrate can be influenced by humidity present due to immersion liquid. Furthermore, not all of the solutions described herein for the liquid supply system may be perfect at containing all of the immersion liquid and some seepage or spillage may occur.

SUMMARY

Accordingly, it would be advantageous, for example, to reduce problems associated with the presence of immersion liquid in a lithographic projection apparatus.

According to an aspect, there is provided a lithographic apparatus comprising:

an illumination system adapted to condition a beam of radiation;

a support structure configured to hold a patterning device, the patterning device configured to impart the beam with a pattern in its cross-section;

a substrate table configured to hold a substrate;

a projection system adapted to project the patterned beam onto a target portion of the substrate;

a liquid supply system configured to at least partly fill a space between the projection system and an object on the substrate table with a liquid; and

an active drying station configured to actively remove the liquid from the object, the substrate table, or both.

Active removal of immersion liquid (which is, in an embodiment, accomplished without substantial heating of the substrate) may ensure that immersion liquid is in contact with the resist on the substrate (if the substrate is the object) for as short a time as possible. This is important because the immersion liquid may react with the resist so that the quality of the image on the substrate can change according to the amount of time that the resist is in contact with immersion liquid. Furthermore, if an active drying station removes liquid from a sensor the performance of that sensor may be enhanced. Removing liquid from an object and/or the substrate table may reduce contamination of the atmosphere in the apparatus with liquid vapor. This may enhance the performance of any optical sensors which may be present in the apparatus. The active removal generally takes place after the object and/or substrate table is moved from under the projection system and/or removed from the liquid supply system i.e. after the supply of liquid to the object and/or substrate table is stopped.

In an embodiment, the substrate table transports the object to the active drying station and supports the object during active removal of liquid from the object by the active drying station. This may ensure that the active removal of liquid takes place as soon as possible after removal from under the projection system (or from the liquid supply system) thereby reducing the amount of time that liquid is exposed to the atmosphere of the apparatus and to the resist on the substrate; the substrate does not need to be removed from the substrate table before it is dried.

The active drying station may be positioned between the projection system and a substrate post exposure processing module so that the removal of liquid can take place between the exposure position and the post exposure processing module or in the case of a substrate table mounted sensor, just before measurement using the sensor. That is the active drying station is within and/or part of the projection apparatus and not the post exposure processing module.

In an embodiment, the drying station comprises gas flow device to provide a flow of gas over a surface of the object and/or the substrate table. In an embodiment, the gas flow device can provide a flow of gas of at least 50 liters per minute. This may ensure that the humidity inside the lithographic apparatus remains low and can help in keeping the apparatus stable in terms of temperature fluctuations, for example. The active drying station may comprise a gas inlet configured to provide gas onto a surface of the object and/or substrate table and/or a gas outlet configured to remove gas and/or liquid from the surface of the object and/or substrate table. In the case of a gas inlet, the gas inlet may comprise a gas knife or a gas shower with at least ten inlets. Both of those solutions have been found to be particularly effective at removing liquid from the surface of the object and/or the substrate table.

The active drying station may comprise a spinner configured to spin the object and/or substrate table. The spinner makes use of centrifugal forces to remove liquid from the object and/or substrate table. This solution may be particularly suitable for when the object is the substrate in which case the substrate is spun in its major plane around its center.

Another alternative which may be used by itself or in addition to any of the other measures discussed above, wherein the liquid between the projection system and an object on the substrate table is a first liquid, comprises a first liquid dissolving liquid supply device configured to provide a second liquid, in which the first liquid dissolves, to a surface of the object. In this way the first liquid can be dissolved in the second liquid. The second liquid itself is chosen for easy removal from the substrate. This may be effected, for example, by choosing a second liquid which has wetting properties of the surface of the object and/or substrate table which promote drying. Alternatively or additionally, the second liquid may be chosen to be more volatile than the first liquid such that it easily evaporates off of the surface of the object and/or substrate table. In an embodiment, the second liquid is a ketone or an alcohol.

According to a further aspect, there is provided a device manufacturing method comprising:

providing a liquid between a projection system and an object on the substrate table;

projecting a patterned beam of radiation, through the liquid, onto a target portion of a substrate using the projection system; and

actively removing the liquid from the object, the substrate table, or both.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a projection beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the projection beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the projection beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

A patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned. In each example of a patterning device, the support structure may be a frame or table, for example, which may be fixed or movable as required and which may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device”.

The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.

The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”.

The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

FIG. 1 depicts a lithographic apparatus according to an embodiment of the invention;

FIG. 2 illustrates an embodiment of the active drying station in which a gas knife is employed;

FIG. 3 illustrates an embodiment of the active drying station in which a gas shower is employed;

FIG. 4 illustrates schematically the principle of a spinner used in an embodiment of the active drying station;

FIG. 5 illustrates, in cross-section, an exemplary embodiment of a gas knife;

FIG. 6 shows, in cross-section, a liquid supply system in accordance with the present invention;

FIG. 7 shows, in plan, the liquid supply system of FIG. 6; and

FIG. 8 depicts a liquid supply system according to an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to a particular embodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL for providing a projection beam PB of radiation (e.g. UV radiation).

a first support structure (e.g. a mask table) MT for supporting a patterning device (e.g. a mask) MA and connected to a first positioner for accurately positioning the patterning device with respect to item PL;

a substrate table (e.g. a wafer table) WT for holding a substrate (e.g. a resist-coated wafer) W and connected to a second positioner for accurately positioning the substrate with respect to item PL; and

a projection system (e.g. a refractive projection lens) PL for imaging a pattern imparted to the projection beam PB by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above).

The illuminator IL receives a beam of radiation from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising for example suitable directing mirrors and/or a beam expander. In other cases the source may be integral part of the apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

The illuminator IL may comprise adjusting means AM for adjusting the angular intensity distribution of the beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL generally comprises various other components, such as an integrator IN and a condenser CO. The illuminator provides a conditioned beam of radiation, referred to as the projection beam PB, having a desired uniformity and intensity distribution in its cross-section.

The projection beam PB is incident on the mask MA, which is held on the mask table MT. Having traversed the mask MA, the projection beam PB passes through the lens PL, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g. an interferometric device), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the beam PB. Similarly, the first positioner and another position sensor (which is not explicitly depicted in FIG. 1) can be used to accurately position the mask MA with respect to the path of the beam PB, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the object tables MT and WT will be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first and/or second positioners. However, in the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2.

The depicted apparatus can be used in the following preferred modes:

-   1. In step mode, the mask table MT and the substrate table WT are     kept essentially stationary, while an entire pattern imparted to the     projection beam is projected onto a target portion C at one time     (i.e. a single static exposure). The substrate table WT is then     shifted in the X and/or Y direction so that a different target     portion C can be exposed. In step mode, the maximum size of the     exposure field limits the size of the target portion C imaged in a     single static exposure. -   2. In scan mode, the mask table MT and the substrate table WT are     scanned synchronously while a pattern imparted to the projection     beam is projected onto a target portion C (i.e. a single dynamic     exposure). The velocity and direction of the substrate table WT     relative to the mask table MT is determined by the (de-)     magnification and image reversal characteristics of the projection     system PL. In scan mode, the maximum size of the exposure field     limits the width (in the non-scanning direction) of the target     portion in a single dynamic exposure, whereas the length of the     scanning motion determines the height (in the scanning direction) of     the target portion. -   3. In another mode, the mask table MT is kept essentially stationary     holding a programmable patterning device, and the substrate table WT     is moved or scanned while a pattern imparted to the projection beam     is projected onto a target portion C. In this mode, generally a     pulsed radiation source is employed and the programmable patterning     device is updated as required after each movement of the substrate     table WT or in between successive radiation pulses during a scan.     This mode of operation can be readily applied to maskless     lithography that utilizes a programmable patterning device, such as     a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

According to an embodiment, the lithographic apparatus is provided with at least one active drying station ADS. In an embodiment, the active drying station ADS is positioned as close as possible to the projection system PL and the liquid supply system LSS which is positioned under the projection system PL. By active drying it is meant that positive measures are taken to remove liquid from an object rather than just performing normal operations on the object during which liquid may evaporate or run off naturally and not applying any measures to the object with the specific aim of drying the object. The liquid supply system LSS may be of any type including a localized area liquid supply system which provides liquid to a localized area on a surface of the substrate as well as to baths in which the whole of the substrate is immersed, etc.

When the substrate W is first moved from under the projection system PL away from the liquid supply system LSS (for example when it is lifted out of a liquid supply system in the form of a bath or when water is drained from that bath) the substrate is taken to the active drying station ADS. At the active drying station ADS immersion liquid which remains on the substrate W is actively removed by one or a combination of the measures described below. The resist (radiation sensitive coating on the substrate) is not affected or removed by the active drying station ADS.

Although the active drying station ADS will be described in relation to removing liquid from the substrate W, the active drying station may also be used for removing immersion liquid from sensors (which may be positioned on the substrate table WT and whose performance will be enhanced by removal of liquid) as well as from the substrate table itself It may be used to remove liquid from other objects too. In the case of sensors, the liquid can be advantageously removed prior to measurement i.e. before exposure of the substrate W.

After having immersion liquid supplied to the substrate W, it is rarely possible to remove all remaining immersion liquid from the surface of the substrate before processing. Unfortunately immersion liquid can dissolve into resist on the substrate as well as the resist dissolving into the immersion liquid. Therefore, it is desirable, with the aim of uniform development of the substrate, to remove the remaining immersion liquid as soon as possible. This is done in the active drying station ADS. The active drying station ADS is, in an embodiment, positioned such that the substrate can be dried within a predetermined time after exposure of the substrate. The predetermined time is, in one or more implementations, less than 5 seconds, less than 2 seconds or less than 1 second. Furthermore, it is disadvantageous for the humidity in the apparatus to be high so that it is advantageous to remove immersion liquid remaining on the substrate table and sensors etc. as soon as possible as well. All of these functions can be performed by the active drying station ADS, in an embodiment, positioned in and/or part of the lithographic projection apparatus. Removal of remaining liquid from the sensors means that liquid cannot disturb proper leveling of the subsequent substrate, which may be a problem.

Although not depicted in FIG. 1, the active drying station ADS may be provided with a baffle or other suitable means to fully enclose the object as it is being dried. For example, the whole of the top surface of the substrate table WT may be enclosed with a curtain or baffle in the active drying station ADS to substantially prevent the spread of immersion liquid around the apparatus. A tray could be used to catch any drips.

Although the active drying station ADS has been illustrated in FIG. 1 as being part of the lithographic apparatus, this is not necessarily the case, and the active drying station ADS may be positioned anywhere between the position at which the substrate W is exposed to the projection beam PB and before a substrate post exposure processing module at which various processing steps such as baking of the resist, development and/or etching are performed. The active drying station may therefore be external of the lithographic projection apparatus.

The active drying station ADS can make use of any means to remove liquid from the substrate W. In an embodiment, the drying is achieved without substantial heating of the substrate W as this could unbalance the apparatus due to the generation of thermal gradients. Several examples of the way in and devices by which the active drying station ADS removes liquid from the substrate W are described below. These methods and/or devices may be used singly or in combination, either at the same time or one after another, as is appropriate.

In an embodiment, the active drying station ADS is positioned within the lithographic projection apparatus and the substrate table WT transports the substrate (or other object) which is to be dried to the active drying station ADS and supports the object (e.g., substrate, sensor, etc.) during active removal of immersion liquid from the object by the active drying station ADS. There may be a plurality of active drying stations, for example, one for the substrate W and one for the sensor(s) on the substrate table WT.

In its simplest form, the active drying station ADS comprises a gas flow device 10 to provide a flow of gas over the surface of the substrate W. The higher the flow of gas the more effective and quick is the removal of immersion liquid from the substrate W. In an embodiment, a flow rate of gas of at least 50 liters per minute, in an implementation, at least 100 liters per minute is achievable. In an embodiment, the gas is filtered and/or dried prior to being provided onto the substrate W to avoid contamination of the surface of the substrate W if this is compatible with the resist (which is not the case for some resists used for 248 nm radiation).

In the embodiment illustrated in FIG. 2, the gas flow device comprises an inlet 50 and an outlet 60. Gas under pressure is provided through inlet 50 and removed by an under pressure in outlet 60. The arrangement illustrated in FIG. 2 is a so-called gas knife in which the inlet 50 is a nozzle which accelerates the gas towards the substrate W so that it impinges on the substrate at both a high flow rate and a high velocity. In an embodiment, the gas jet impinges on the substrate at an acute angle to the substrate W in a direction towards the outlet 60. Such an arrangement is particularly effective at removing liquid from the surface of the substrate W. The gas flow device 10 may comprise only a single inlet 50 or a single outlet 60 or may comprise a plurality of in and outlets 50, 60. The substrate W may be moved under stationary in and outlet(s) 50, 60 as indicated by arrow 15. The in and outlets 50, 60 may also be a continuous groove as well as discrete in and outlet ports. The in and outlets 50, 60 may also be moveable.

It is also possible for the gas flow device only to comprise an outlet 60 which is connected to an under pressure. In this case gas as well as immersion liquid on top of the substrate W will be sucked up through the outlet 60.

A further embodiment is illustrated in FIG. 3 in which the gas flow device 10 comprises a plurality of inlets 50 and no outlets above the substrate (although outlets may need to be provided elsewhere to drain off the excess gas). This is a so-called shower head. In an embodiment, the shower head comprises at least ten inlets 50. The shower head may be of a cross-sectional area which is large enough to cover the whole or part of the surface of the substrate W. The substrate W may be moved under the shower head 10 as it dries.

In an embodiment, the active drying station ADS is dimensioned to be as long as the substrate table WT so that all objects on the substrate table WT (including the substrate and sensor(s)) can be dried at one time.

Again, in an implementation, filtered gas is used in the shower head 60.

A particularly effective form of gas knife for removing liquid from the substrate W or sensor(s) is shown in FIG. 5. The immersion liquid 5 is sucked up a central passage 110 which is connected to an under pressure. The passage 110 is, in an embodiment, in the form of a slot (extending in and out of the page). Gas is supplied from an outer passage 120 on either, some or all sides of the central passage. The outer passage 120 may also be one or more slots. The outer passage 120 may a single passage that peripherally extends around the central passage 110. Thus, there is a gas flow over the surface of the substrate W which helps draw gas and immersion liquid 5 up into central passage 110. The outer passage may be directed at its outlet at an angle to the surface of the substrate away from perpendicular and towards the inlet of the central passage 110. The bottom surface of the gas knife in which outlet(s) and inlet(s) of the passages 110, 120 are formed may be contoured so that gas flow from the outer passage 120 to the central passage 110 is smooth e.g. by rounding off the edges (i.e. giving a radius to the edges) of the inlet to the central passage 110 and optionally the outlet(s) of the outer passage 120.

In an embodiment, the active drying station ADS is positioned and the path of the substrate table WT chosen such that the substrate table WT moves under the active drying station ADS during the substrate table's normal movement after or before exposure (i.e. no deviation of path way may be required). Thus, through put penalty is minimized.

A further system which may be used in the active drying station ADS is a spinner which is used for spinning the substrate W in the plane of the substrate W around its central point. When the substrate W is spinning (as is illustrated in FIG. 4) centrifugal forces act on the liquid on the surface of the substrate W which is flung outwards where the immersion liquid may be collected.

In an embodiment, the active drying station ADS comprises a liquid retrieval device to recover immersion fluid removed from the object being dried. This is particularly advantageous if the immersion liquid is not water.

In a further embodiment which can be used in combination with any one of the foregoing embodiments, by being positioned before or after the foregoing embodiments, is to use a drying liquid in which the immersion liquid dissolves on the surface of the substrate W. If the drying liquid in which the immersion liquid dissolves is of a type more easily removed from the surface of the substrate than the immersion liquid, this may speed up the drying process. Furthermore, by dissolving the immersion liquid, it may be possible to reduce dissolution of the resist or diffusion into the resist by careful choice of the drying liquid which is used. Thus, an immersion liquid dissolving liquid supply device is provided which can provide drying liquid, in which the immersion liquid dissolves, to the surface of the substrate W. The drying liquid chosen for this task is, in an implementation, more volatile than the immersion liquid such that it can be removed (i.e. evaporated) more easily than the immersion liquid. Alternatively or additionally the liquid may be chosen such that it has a high contact angle with the substrate W so that it beads more easily than the immersion liquid on the substrate W and can therefore be removed. Suitable liquids are ketones or alcohols, in particular IPA (isopropylalchohol).

It will be appreciated that the present invention has been described in relation to the substrate W being dried by the active drying station ADS while in position on the substrate table WT. This is not necessarily the case and other objects such as the substrate table WT itself or sensors on the substrate table WT may be dried by the active drying station. Drying of a sensor on the substrate which may be immersed in an immersion liquid on a surface of the sensor (by accident or for illumination, for example) is advantageous. The performance of the sensors is improved by elimination of liquid during measurements and/or elimination of drying marks. Furthermore, it may be necessary to remove the substrate W from the substrate table WT before being dried by the active drying station ADS, which as stated above, may be positioned outside of the lithographic apparatus. Indeed, even if the active drying station is positioned in the lithographic apparatus, it may be necessary for engineering issues to remove the substrate/object from the substrate table for drying by the active drying station ADS.

Another immersion lithography solution which has been proposed is to provide the liquid supply system with a seal member which extends along at least a part of a boundary of the space between the final element of the projection system and the substrate table. The seal member is substantially stationary relative to the projection system in the XY plane though there may be some relative movement in the Z direction (in the direction of the optical axis). A seal is formed between the seal member and the surface of the substrate. In an implementation, the seal is a contactless seal such as a gas seal. Such a system is disclosed in, for example, U.S. patent application Ser. No. 10/705,783, hereby incorporated in its entirety by reference.

A further immersion lithography solution with a localized liquid supply system is shown in FIG. 8. Liquid is supplied by two groove inlets IN on either side of the projection system PL and is removed by a plurality of discrete outlets OUT arranged radially outwardly of the inlets IN. The inlets IN and OUT can be arranged in a plate with a hole in its center and through which the projection beam is projected. Liquid is supplied by one groove inlet IN on one side of the projection system PL and removed by a plurality of discrete outlets OUT on the other side of the projection system PL, causing a flow of a thin film of liquid between the projection system PL and the substrate W. The choice of which combination of inlet IN and outlets OUT to use can depend on the direction of movement of the substrate W (the other combination of inlet IN and outlets OUT being inactive).

In European patent application no. 03257072.3, hereby incorporated in its entirety by reference, the idea of a twin or dual stage immersion lithography apparatus is disclosed. Such an apparatus is provided with two substrate tables for supporting the substrate. Leveling measurements are carried out with a substrate table at a first position, without immersion liquid, and exposure is carried out with a substrate table at a second position, where immersion liquid is present. Alternatively, the apparatus can have only one substrate table moving between the first and second positions.

Embodiments of the present invention may be applied to any immersion lithography apparatus and any liquid supply system (including relevant parts thereof), in particular, but not exclusively, to any of those liquid supply systems mentioned above and the bath of liquid as described above.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The description is not intended to limit the invention. 

1. An active drying station configured to actively remove immersion liquid from an object in an immersion lithographic apparatus, the station comprising: a gas flow supply configured to supply a flow of gas over a surface of the object, wherein the gas flow supply comprises gas inlets to provide gas at two positions over the surface of the object and a gas outlet positioned between the gas inlets to remove the gas from a location between the two positions.
 2. The station of claim 1, wherein the flow of gas forms a gas knife.
 3. The station of claim 1, wherein the gas inlets are directed at an angle to the surface of the object and towards the gas outlet.
 4. The station of claim 1, further comprising an immersion liquid dissolving liquid supply configured to provide a liquid, in which the immersion liquid dissolves, to the surface of the object.
 5. The station of claim 1, wherein the object is a substrate, a substrate table or a sensor.
 6. The station of claim 1, wherein the object is positioned on a substrate table of the immersion lithographic apparatus and wherein, in use, the immersion liquid at least partly fills a space between a projection system of the immersion lithographic apparatus and the substrate table.
 7. The station of claim 6, wherein the substrate table is configured to hold a substrate during projection of a patterned beam of radiation by the projection system on the substrate table, the substrate table being configured to transport the object to the station.
 8. The station of claim 1, wherein the gas inlets substantially surround the periphery of the gas outlet.
 9. The station of claim 1, wherein the station is constructed and arranged to be external of the immersion lithography apparatus.
 10. The station of claim 1, wherein the station is arranged between a projection system of the immersion lithographic apparatus and a substrate post exposure processing module.
 11. The station of claim 1, wherein at least part of the immersion liquid is removed by the active drying station from under a projection system of the immersion lithographic apparatus.
 12. An active drying method comprising: receiving an object with immersion liquid thereon from an immersion lithographic apparatus; and removing at least part of the immersion liquid from a surface of the object by supplying a flow of gas over the surface of the object with a gas flow supply, wherein the gas flow supply comprises gas inlets to provide gas at two positions over the surface of the object and a gas outlet positioned between the gas inlets to remove the gas from a location between the two positions.
 13. The method of claim 12, wherein the flow of gas forms a gas knife.
 14. The method of claim 12, wherein the flow of gas created by the gas flow supply is provided at an angle to the surface of the object.
 15. The method of claim 12, further comprising providing a dissolving liquid, in which the immersion liquid dissolves, to a surface of the object.
 16. The method of claim 12, wherein the object is a substrate, a substrate table or a sensor.
 17. The method of claim 12, wherein the gas inlets substantially surround the periphery of the gas outlet.
 18. The method of claim 12, wherein the object is positioned on a substrate table of the immersion lithographic apparatus and wherein, in use, the immersion liquid at least partly fills a space between a projection system of the immersion lithographic apparatus and the substrate table.
 19. The method of claim 18, wherein the flow of gas is supplied from under the projection system. 